So-called protective devices which monitor lines for faults are used to protect the operation of power supply lines. One such protective device is, for example, a distance protection device. Distance protection measures the impedance of the line and monitors this line for undershooting an impedance value which can be predetermined. Undershooting occurs, for example, in the case of short circuits.
A problem with modern distance protective devices of digital design is that filters are used which have a fixed window width and a fixed release criterion. In this case, there is a contradiction between the two criteria "high accuracy of impedance determination" and "short command time for the off command". Specifically, if a protection algorithm is to operate quickly at a fixed technically sensible sampling frequency, then digital filters having few coefficients must be used. When disturbance variables occur, such filters have large errors in the impedance determination, which leads to a disturbance in the selectivity. In contrast, if a protection algorithm is to operate as selectively as possible, then filters having large numbers of coefficients must be used to damp interference variables. The protection algorithm then, however, has relatively long command times. This problem is known, for example, from "Proceedings of the 25th Universities Power Engineering Conference", Aberdeen, UK, Sep. 12-14, 1990, pages 155 to 158.
A test method is known from EP 0 284 546, in which filters having a fixed window width are used.